Edge couplers with a partially-etched inverse taper

ABSTRACT

Structures including an edge coupler and methods of fabricating a structure including an edge coupler. The edge coupler includes a waveguide core having an end surface and a tapered section that terminates at the end surface. The tapered section of the waveguide core includes a slab layer and a ridge layer on the slab layer. The slab layer and the ridge layer each terminate at the end surface. The slab layer has a first width dimension with a first width at a given location along a longitudinal axis of the waveguide core, the ridge layer has a second width dimension with a second width at the given location along the longitudinal axis of the waveguide core, and the first width is greater than the second width.

BACKGROUND

The present invention relates to photonics chips and, more specifically,to structures including an edge coupler and methods of fabricating astructure including an edge coupler.

Photonics chips are used in many applications and systems such as datacommunication systems and data computation systems. A photonics chipintegrates optical components, such as waveguides, optical switches,edge couplers, and polarizers, and electronic components, such asfield-effect transistors, into a unified platform. Among other factors,layout area, cost, and operational overhead may be reduced by theintegration of both types of components.

An edge coupler is commonly used for coupling laser light between asemiconductor laser and optical components on the photonics chip. Theedge coupler may include a narrowed section of a waveguide core that hasa significantly smaller mode size than the beam of laser light emittedby the semiconductor laser. Inefficiencies in the coupling between thesemiconductor laser and the edge coupler may occur due to the mismatchedmode size, as well as differences in mode shape. These inefficienciesmay give rise to a significant coupling loss.

Improved structures including an edge coupler and methods of fabricatinga structure including an edge coupler are needed.

SUMMARY

In an embodiment of the invention, a structure includes an edge couplerhaving a waveguide core with an end surface and a tapered section thatterminates at the end surface. The tapered section of the waveguide coreincludes a slab layer and a ridge layer on the slab layer. The slablayer and the ridge layer each terminate at the end surface. The slablayer has a first width dimension at a given location along thelongitudinal axis of the waveguide core, the ridge layer has a secondwidth dimension at the given location along the longitudinal axis of thewaveguide core, and the first width dimension is greater than the secondwidth dimension.

In an embodiment of the invention, a method of forming a structure foran edge coupler is provided. The method includes forming a waveguidecore that includes an end surface and a tapered section that terminatesat the end surface. The tapered section of the waveguide core includes aslab layer and a ridge layer on the slab layer, and the slab layer andthe ridge layer each terminate at the end surface. The slab layer has afirst width dimension at a given location along the longitudinal axis ofthe waveguide core, the ridge layer has a second width dimension at thegiven location along the longitudinal axis of the waveguide core, andthe first width dimension is greater than the second width dimension.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate various embodiments of theinvention and, together with a general description of the inventiongiven above and the detailed description of the embodiments given below,serve to explain the embodiments of the invention. In the drawings, likereference numerals refer to like features in the various views.

FIG. 1 is a top view of a structure for an edge coupler at an initialfabrication stage of a processing method in accordance with embodimentsof the invention.

FIG. 2 is a cross-sectional view of the structure taken generally alongline 2-2 in FIG.

FIG. 3 is a cross-sectional view of the structure at a fabrication stagesubsequent to FIG. 2.

FIGS. 4-6 are top views of structures in accordance with alternativeembodiments of the invention.

FIG. 7 is a cross-sectional view of a structure in accordance withalternative embodiments of the invention.

DETAILED DESCRIPTION

With reference to FIGS. 1, 2 and in accordance with embodiments of theinvention, a structure 10 for an edge coupler includes a waveguide core12 having a tapered section 14, an end surface 16 that is positionedadjacent to a laser 20, and a non-tapered section 18 adjoined andconnected to the tapered section 14. In an embodiment, the non-taperedsection 18 of the waveguide core 12 may be a straight section locatedimmediately adjacent to the tapered section 14. The waveguide core 12may be extend axially with alignment along a longitudinal axis 17. Inthe representative embodiment, the end surface 16 terminates the taperedsection 14 and provides a surface of given cross-sectional area that isbutted with the laser 20 and that participates in receiving laser lightemitted from the laser 20. The tapered section 14, which is inverselytapered, may extend lengthwise parallel to the longitudinal axis 17 fromthe end surface 16 to an opposite end at a transition to the non-taperedsection 18. The tapered section 14 has a length measured as a distancealong the longitudinal axis 17 from the end surface 16 to the transitionbetween the tapered section 14 and the non-tapered section 18.

The tapered section 14 and non-tapered section 18 of the waveguide core12 include respective portions of a slab layer 26 and a ridge layer 28.The ridge layer 28 is positioned on and over the slab layer 26 to definea rib-type waveguiding structure. The slab layer 26 has a thickness, t1,that is less than a thickness, t2, of the ridge layer 28. The waveguidecore 12 has a top surface 11 and a bottom surface opposite to the topsurface 11 in a vertical direction, and the thickness, t2, of the ridgelayer 28 may be measured from the top surface 11 to the bottom surface.Over the length of the tapered section 14, the slab layer 26 includesopposite sidewalls or side surfaces 30, 32 that diverge with increasingdistance from the end surface 16. The ridge layer 28 of the waveguidecore 12 also includes opposite sidewalls or side surfaces 22, 24 thatdiverge with increasing distance from the end surface 16 over the lengthof the tapered section 14. The separation between the side surfaces 22,24 of the ridge layer 28 may be constant in the non-tapered section 18and, similarly, the separation between the side surfaces 30, 32 of theslab layer 26 may be constant in the non-tapered section 18.

In the tapered section 14, the slab layer 26 has a width dimension thatincreases with increasing distance from the end surface 16 and definesan inverse taper characterized by a taper angle, θ. The increasing widthdimension of the slab layer 26 of the waveguide core 12 over the lengthof the tapered section 14 may end at the transition to the non-taperedsection 18. In the tapered section 14, the ridge layer 28 likewise has awidth dimension that increases with increasing distance from the endsurface 16 and defines an inverse taper characterized by a taper angle,φ. The increasing width dimension of the ridge layer 28 of the waveguidecore 12 over the length of the tapered section 14 may end at thetransition to the non-tapered section 18.

The ridge layer 28 of the waveguide core 12 may have a width, W1,between the side surfaces 22, 24 at the end surface 16, and the ridgelayer 28 of the waveguide core 12 may have a larger width, W2, betweenthe side surfaces 22, 24 at the transition from the tapered section 14to the non-tapered section 18. The width, W1, of the ridge layer 28 mayrange from 0.01 times the wavelength of the laser light received fromthe laser 20 to 0.1 times the wavelength of the laser light receivedfrom the laser 20. The width, W2, of the ridge layer 28 may be greaterthan 0.25 times the wavelength of the laser light received from thelaser 20.

At any given location along the longitudinal axis 17 in the taperedsection 14, the ridge layer 28 may have a width dimension that variesbetween a minimum width equal to the width, W1, and a maximum widthequal to the width, W2. In an embodiment, the width dimension of theridge layer 28 may linearly vary between the width, W1, and the width,W2, based on a linear function such that the taper angle, φ, isconstant. In an alternative embodiment, the ridge layer 28 may have awidth dimension that non-linearly varies between the width, W1, and thewidth, W2, based on a non-linear function, such as a quadratic,parabolic, or exponential function, over its length such that the taperangle, φ, varies.

The slab layer 26 of the waveguide core 12 may have a width, W3, betweenthe side surfaces 30, 32 at the end surface 16, and the slab layer 26 ofthe waveguide core 12 may have a larger width, W4, between the sidesurfaces 30, 32 at the transition from the tapered section 14 to thenon-tapered section 18. At any given location along the longitudinalaxis 17 in the tapered section 14, the slab layer 26 may have a widthdimension that varies between a minimum width given by the width, W3,and a maximum width given by the width, W4. In an embodiment, the widthdimension of the slab layer 26 may linearly vary between width, the W3,and the width, W4, based on a linear function such that the taper angle,θ, is constant. In an alternative embodiment, the slab layer 26 may havea width dimension that non-linearly varies between the width, W3, andthe width, W4, based on a non-linear function, such as a quadratic,parabolic, or exponential function, over its length such that the taperangle, θ, varies.

At the end surface 16, the width, W3, of the slab layer 26 is greaterthan the width, W1, of the ridge layer 28. At the transition from thetapered section 14 to the non-tapered section 18, the width, W4, of theslab layer 26 is greater than the width, W2, of the ridge layer 28. Atany given location along the longitudinal axis 17 in the tapered section14, the width dimension of the slab layer 26 is greater than the widthdimension of the ridge layer 28. The larger width dimension of the slablayer 26 in comparison with the ridge layer 28 continues into thenon-tapered section 18. In particular, the slab layer 26 may have thewidth, W4, and the ridge layer 28 may have the width, W2, in thenon-tapered section 18.

In the representative embodiment, the taper angle, θ, of the inversetaper defined by the slab layer 26 may be equal to the taper angle, φ,of the inverse taper defined by the ridge layer 28. In such anembodiment with equal taper angles, the difference in the widthdimension of the ridge layer 28 and the width dimension of the slablayer 26 is constant over the length of the tapered section 14. In anembodiment, the width dimensions of the slab layer 26 and the ridgelayer 28 in the tapered section 14 may both vary linearly based on alinear function. In an embodiment, the slab layer 26 may besymmetrically arranged relative to the ridge layer 28 such that equalwidths of the slab layer 26 are located adjacent to each of the sidesurfaces 22, 24 of the ridge layer 28 for linear width variations andequal taper angles.

In an alternative embodiment, the width dimensions of the slab layer 26and the ridge layer 28 in the tapered section 14 may both varynon-linearly based on a non-linear function, such as a quadraticfunction or an exponential function. In an alternative embodiment, thewidth dimension of the slab layer 26 in the tapered section 14 may varylinearly based on a linear function, and the width dimension of theridge layer 28 may vary non-linearly based on a non-linear function. Inan alternative embodiment, the width dimension of the slab layer 26 inthe tapered section 14 may vary non-linearly based on a non-linearfunction, and the width dimension of the ridge layer 28 may varylinearly based on a linear function.

The waveguide core 12 may be arranged over a dielectric layer 34. Thewaveguide core 12 may be comprised of a single-crystal semiconductormaterial, such as single-crystal silicon. In an embodiment, thesingle-crystal semiconductor material may originate from a device layerof a silicon-on-insulator (SOI) wafer that further includes a buriedoxide layer providing the dielectric layer 34 and a substrate 36comprised of a single-crystal semiconductor material, such assingle-crystal silicon. The waveguide core 12 may be patterned from alayer of the single-crystal semiconductor material by lithography andetching processes during front-end-of-line processing of a photonicschip. In an embodiment, multiple patterning processes may be used toform the slab layer 26 and the ridge layer 28. For example, an initialetch mask having the shape of the slab layer 26 may be formed over thelayer of the single-crystal semiconductor material and an etchingprocess may be used to reproduce the shape in the layer by etching fullythrough the layer to the dielectric layer 34. Another etch mask havingthe shape of the ridge layer 28 may then be formed over thepreviously-etched layer of the single-crystal semiconductor material andanother etching process may be used to etch partially through thepreviously-etched layer to define the slab layer 26.

In an alternative embodiment, the waveguide core 12 may be comprised ofsilicon nitride, silicon oxynitride, or aluminum nitride instead ofsingle-crystal silicon. In an alternative embodiment, the substrate 36may include a groove extending beneath the dielectric layer 34 as anunder-cut immediately adjacent to the end surface 16.

The laser 20 may be configured to emit laser light of a givenwavelength, intensity, mode shape, and mode size that is directed towardthe end surface 16 of the waveguide core 12. The space between the laser20 and the end surface 16 of the waveguide core 12 may be filled by airor, alternatively, may be filled by an index-matching material. Thelaser 20 may be comprised of III-V compound semiconductor materials. Forexample, the laser 20 may be an indium phosphide/indium-gallium-arsenicphosphide laser that is configured to generate and output continuouslaser light in an infrared wavelength range. For example, the laser 20may generate and output laser light at a nominal peak wavelength of 1310nm or at a nominal peak wavelength of 1550 nm. The laser 20 may belocated on the photonics chip including the waveguide core 12 or,alternatively, the laser 20 may be located off-chip. The laser 20 may beseparately manufactured and may be attached to surfaces surrounding acavity formed in the substrate 36 by, for example, flip-chip bonding.

With reference to FIG. 3 in which like reference numerals refer to likefeatures in FIGS. 1, 2 and at a subsequent fabrication stage, adielectric layer 38 is formed over the waveguide core 12. The dielectriclayer 38 may be comprised of a dielectric material, such as silicondioxide, deposited by chemical vapor deposition and planarized by, forexample, chemical-mechanical polishing to remove topography. Below itstop surface 11, the waveguide core 12 is embedded in the dielectricmaterial of the dielectric layer 38. Additional dielectric layers (notshown) may be formed over the dielectric layer 38 and, for example, theadditional dielectric layers may be respectively comprised of siliconnitride and silicon dioxide to define a heterogeneous dielectric layerstack.

A back-end-of-line stack 40 is formed over the dielectric layer 38. Theback-end-of-line stack 40 includes one or more interlayer dielectriclayers that may be comprised of dielectric material, such as silicondioxide, and metallization comprised of, for example, copper oraluminum, that is arranged in the one or more interlayer dielectriclayers.

The structure 10, in any of its embodiments described herein, may beintegrated into a photonics chip that includes electronic components andadditional optical components. For example, the electronic componentsmay include field-effect transistors that are fabricated by CMOSfront-end-of-line (FEOL) processing.

The structure 10 provides a monolithically-integrated edge coupler forefficient butt-end light coupling with the laser 20. The structure 10may improve mode matching (i.e., the matching of mode shape and/or modesize) with the laser output to enhance the efficiency of the lightcoupling. The addition of the slab layer 26 may reduce the coupling lossand back reflection exhibited by the end coupler, and may improve thetransmission efficiency and power handling capability exhibited by theend coupler. The structure 10 may permit the laser light to be coupledwith active optical components of the photonics chip, such asmodulators, without the need for an additional transition region betweenthe non-tapered section 18 and the active optical components.

With reference to FIG. 4 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with alternative embodiments of theinvention, the taper angle, θ, of the inverse taper defined by the slablayer 26 and the taper angle, φ, of the inverse taper defined by theridge layer 28 may be unequal. In the representative embodiment, thetaper angle, θ, of the inverse taper defined by the slab layer 26 may begreater than the taper angle, φ, of the inverse taper defined by theridge layer 28, and the taper angle difference increases the differencebetween the width, W4, and the width, W2.

With reference to FIG. 5 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with alternative embodiments of theinvention, the tapering of the slab layer 26 in the tapered section 14of the waveguide core 12 may include multiple taper angles, and thetapering of the ridge layer 28 in the tapered section 14 of thewaveguide core 12 may also include multiple taper angles. In eachinstance, the multiple taper angles provide stages of a compound taperfor the slab layer 26 and ridge layer 28 in the tapered section 14 ofthe waveguide core 12. In the representative embodiment, the slab layer26 narrows with a taper angle in a portion 14 a of the tapered section14 adjacent to the end surface 16 and with a larger taper angle in aportion 14 b of the tapered section 14. In the representativeembodiment, the ridge layer 28 likewise narrows with a taper angle in aportion 14 a of the tapered section 14 adjacent to the end surface 16and with a larger taper angle in a portion 14 b of the tapered section14.

The lengths of the slab layer 26 and ridge layer 28 in the portion 14 aof the tapered section 14 may be equal. The lengths of the slab layer 26and ridge layer 28 in the portion 14 b of the tapered section 14 mayalso be equal. In an alternative embodiment, the lengths of the taperedportions 14 a, 14 b of the slab layer 26 in the tapered section 14 maydiffer from the lengths of the tapered portions 14 a, 14 b of ridgelayer 28 in the tapered section 14.

With reference to FIG. 6 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with alternative embodiments of theinvention, the slab layer 26 may include a transition region 42 thatterminates the slab layer 26 at an end opposite to the terminationprovided by the end surface 16. The transition region 42 provides acoupler transitioning the waveguide core 12 from the rib-typewaveguiding structure to a different type of waveguiding structure. Thetransition region 42 provides a transition from the tapered section 14of the waveguide core 12 to a section of the waveguide core 12 in whichthe ridge layer 28 is present and the slab layer 26 is absent. The slablayer 26 in the tapered section 14 of the waveguide core 12 islongitudinally positioned along the longitudinal axis 17 between thetransition region 42 of the slab layer 26 and the end surface 16. In anembodiment, the transition region 42 may be a taper that narrows in anopposite direction along the longitudinal axis 17 from the narrowing(i.e., inverse tapering) of the slab layer 26 in the tapered section 14.In an alternative embodiment, the ridge layer 28 may be coupled by anoptical coupler (not shown) as a transition region that transfers thelaser light upward to an overlying ridge waveguide core (not shown),such as an overlying ridge waveguide core comprised of silicon nitride.

With reference to FIG. 7 in which like reference numerals refer to likefeatures in FIG. 1 and in accordance with alternative embodiments of theinvention, an additional slab layer 27 may be formed between the slablayer 26 and the ridge layer 28 to provide a composite slab layer. Theslab layer 26 is positioned between the slab layer 27 and the dielectriclayer 34, and the slab layer 27 is positioned between the ridge layer 28and the slab layer 26. The slab layer 27 has a thickness, t3, that maybe equal to or differ from the thickness, t1, of the slab layer 26. Theslab layers 26, 27 each have a thickness that is less than thethickness, t2, of the ridge layer 28. The width dimension of the slablayer 26 is greater than the width dimension of the slab layer 27, andthe width dimension of the ridge layer 28 is less than the widthdimension of either the slab layer 26 or the slab layer 27. Thestructure 10 with multiple slab layers 26, 27 may provide a shape thatimproves the coupling with the laser 20.

References herein to terms modified by language of approximation, suchas “about”, “approximately”, and “substantially”, are not to be limitedto the precise value specified. The language of approximation maycorrespond to the precision of an instrument used to measure the valueand, unless otherwise dependent on the precision of the instrument, mayindicate +/−10% of the stated value(s).

References herein to terms such as “vertical”, “horizontal”, etc. aremade by way of example, and not by way of limitation, to establish aframe of reference. The term “horizontal” as used herein is defined as aplane parallel to a conventional plane of a semiconductor substrate,regardless of its actual three-dimensional spatial orientation. Theterms “vertical” and “normal” refer to a direction perpendicular to thehorizontal, as just defined. The term “lateral” refers to a directionwithin the horizontal plane.

A feature “connected” or “coupled” to or with another feature may bedirectly connected or coupled to or with the other feature or, instead,one or more intervening features may be present. A feature may be“directly connected” or “directly coupled” to or with another feature ifintervening features are absent. A feature may be “indirectly connected”or “indirectly coupled” to or with another feature if at least oneintervening feature is present. A feature “on” or “contacting” anotherfeature may be directly on or in direct contact with the other featureor, instead, one or more intervening features may be present. A featuremay be “directly on” or in “direct contact” with another feature ifintervening features are absent. A feature may be “indirectly on” or in“indirect contact” with another feature if at least one interveningfeature is present.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration but are not intended tobe exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

1. A structure comprising: an edge coupler including a waveguide corecomprising single-crystal silicon, the waveguide core having alongitudinal axis, an end surface, and a tapered section that terminatesat the end surface, the tapered section of the waveguide core includinga first slab layer and a ridge layer on the first slab layer, the firstslab layer and the ridge layer each terminating at the end surface, thefirst slab layer having a first width dimension with a first width at agiven location along the longitudinal axis of the waveguide core, theridge layer having a second width dimension with a second width at thegiven location along the longitudinal axis of the waveguide core, andthe first width greater than the second width; and a laser positionedadjacent to the end surface of the waveguide core.
 2. (canceled)
 3. Thestructure of claim 1 wherein the first width dimension of the first slablayer in the tapered section increases with increasing distance from theend surface.
 4. The structure of claim 3 wherein the second widthdimension of the ridge layer in the tapered section increases withincreasing distance from the end surface.
 5. The structure of claim 4wherein the first slab layer and the ridge layer have equal taper anglesin the tapered section.
 6. The structure of claim 4 wherein the firstslab layer and the ridge layer have unequal taper angles in the taperedsection.
 7. The structure of claim 4 wherein the ridge layer has a firsttaper angle in the tapered section, the first slab layer has a secondtaper angle in the tapered section, and the second taper angle isgreater than the first taper angle.
 8. The structure of claim 4 whereinthe ridge layer includes a first plurality of taper angles in differentportions of the tapered section, and the ridge layer includes a secondplurality of taper angles in different portions of the tapered section.9. The structure of claim 1 wherein the waveguide core includes anon-tapered section adjoined to the tapered section, and the taperedsection and the non-tapered section are positioned along thelongitudinal axis with the tapered section between the non-taperedsection and the end surface.
 10. The structure of claim 9 wherein thetapered section and the non-tapered section each include the first slablayer and the ridge layer.
 11. The structure of claim 9 wherein thefirst slab layer and the ridge layer extend along a full length of thetapered section from the end surface to a transition with thenon-tapered section.
 12. (canceled)
 13. The structure of claim 1 whereinthe ridge layer has a first length along the longitudinal axis in thetapered section, the first slab layer has a second length along thelongitudinal axis in the tapered section, and the second length is equalto the first length.
 14. The structure of claim 1 further comprising: asubstrate including a dielectric layer, wherein the waveguide core ispositioned on the dielectric layer.
 15. The structure of claim 1 whereinthe tapered section of the waveguide core includes a second slab layerpositioned between the ridge layer and the first slab layer, the secondslab layer terminates at the end surface, and the second slab layer hasa third width dimension that is greater than the second width dimensionand less than the first width dimension.
 16. The structure of claim 1wherein the first slab layer includes a transition region, and thetapered section of the waveguide core is positioned along thelongitudinal axis between the transition region and the end surface. 17.A method of forming a structure for an edge coupler, the methodcomprising: forming a waveguide core that includes a longitudinal axis,an end surface, and a tapered section that terminates at the endsurface; and positioning a laser adjacent to the end surface of thewaveguide core, wherein the waveguide core comprises single-crystalsilicon, the tapered section of the waveguide core includes a slab layerand a ridge layer on the slab layer, the slab layer and the ridge layereach terminate at the end surface, the slab layer has a first widthdimension with a first width at a given location along the longitudinalaxis of the waveguide core, the ridge layer has a second width dimensionwith a second width at the given location along the longitudinal axis ofthe waveguide core, and the first width is greater than the secondwidth.
 18. The method of claim 17 wherein the slab layer and the ridgelayer are patterned by multiple lithography and etching processes. 19.(canceled)
 20. The method of claim 17 wherein the first width dimensionof the slab layer in the tapered section increases with increasingdistance from the end surface, and the second width dimension of theridge layer in the tapered section increases with increasing distancefrom the end surface.
 21. The structure of claim 1 wherein the laser andthe edge coupler are located on a photonics chip.
 22. The structure ofclaim 1 wherein the laser comprises III-V compound semiconductormaterials.
 23. The structure of claim 14 wherein the laser is attachedto the substrate.